Add GetStringPtr

Returns a string pointer. Returns `nil` if GetString() would have returned
`""`. If you want a pointer to the empty string, assign the string returned by
GetString and take its pointer:

```golang
v = viper.GetString("key")
ptr := &v
```

README.md

@@ -507,6 +507,7 @@ The following functions and methods exist:
  * `GetInt(key string) : int`
  * `GetIntSlice(key string) : []int`
  * `GetString(key string) : string`
+ * `GetStringPtr(key string) : *string`
  * `GetStringMap(key string) : map[string]interface{}`
  * `GetStringMapString(key string) : map[string]string`
  * `GetStringSlice(key string) : []string`
@@ -517,7 +518,7 @@ The following functions and methods exist:
 
 One important thing to recognize is that each Get function will return a zero
 value if it’s not found. To check if a given key exists, the `IsSet()` method
-has been provided.
+has been provided. `GetStringPtr()`, though, will return nil for empty string.
 
 Example:
 ```go

viper.go

@@ -793,6 +793,16 @@ func (v *Viper) GetString(key string) string {
 	return cast.ToString(v.Get(key))
 }
 
+// GetStringPt returns the value associated with the key as a string pointer, nil if it's empty string.
+func GetStringPtr(key string) *string { return v.GetStringPtr(key) }
+func (v *Viper) GetStringPtr(key string) *string {
+	s := v.GetString(key)
+	if s == "" {
+		return nil
+	}
+	return &s
+}
+
 // GetBool returns the value associated with the key as a boolean.
 func GetBool(key string) bool { return v.GetBool(key) }
 func (v *Viper) GetBool(key string) bool {

viper_test.go

@@ -2159,3 +2159,14 @@ func BenchmarkGetBoolFromMap(b *testing.B) {
 		}
 	}
 }
+
+func TestGetStringPtr(t *testing.T) {
+	v := New()
+	v.SetConfigType("yaml")
+
+	/* config and defaults */
+	v.ReadConfig(bytes.NewBuffer(yamlExample))
+
+	assert.Equal(t, "leather", *v.GetStringPtr("clothing.jacket"))
+	assert.Nil(t, v.GetStringPtr("motorcycle"))
+}